           destLessonGroup (data $helper LESSON-GROUP-CHANGE-KEY)
 
           affectedDaysSet
-          (reduce (fn [daySet ttLessonInfo]
-                    (conj daySet (:day ttLessonInfo)))
-                  #{}
-                  Lessons-Created-By-Draggable)]
+          (get-set-from-seq-of-maps-with-day-key Lessons-Created-By-Draggable)]
 
       (doseq [augTTLessonInfo Lessons-Created-By-Draggable]
         (timetable-remove-lesson! (:day augTTLessonInfo)

      (if (not (nil? destLessonGroup))
          (let [augTTLessonInfoSeq (remove-lesson-group-html moduleCode
                                                             lessonType)
                affectedDaysSet    (reduce (fn [daySet augTTLessonInfo]
                                             (conj daySet
                                                   (:day augTTLessonInfo)))
                                           #{}
                                           augTTLessonInfoSeq)]
            ; remove the current lesson group
            (doseq [augTTLessonInfo augTTLessonInfoSeq]
              (let [day          (:day augTTLessonInfo)

                          lessonTypes))

            affectedDaysSet
            (get-set-from-seq-of-maps-with-day-key augTTLessonInfoSeq)]

        ; Remove module from `ModulesSelected`
        (set! ModulesSelected (dissoc ModulesSelected moduleCode))
